Centrifugal compressor



' sages becomes too small.

Psa-ated Apr. es, 1946 UNITED sTATas PATENT. OFFICE CENTRWUGAL COMPRESSOR A Wendell S. Thompson, Lynn, Mass., assignor to General Electric Com New York pany, a corporation oi Application october 1s, 194e, serial No. 559,261

s claims. (crean-127) The present invention relates to centrifugal compressors having impellers for impelling medium to be compressed and dlusers for converting velocity energy of the medium discharged from the impeller into pressure energy and a discharge chamber or scroll for receiving medium from the diffuser and discharging it to a-.consumen Difculties are experienced with this type of compressor due to pulsations and consequent instability at low ow or load. The basic cause of such pulsations is believed to be the complete breakdown and reversal of the boundary layer in the diffuser passages occurring when the volume of ow through such paslow ow may be reduced and the operating range of the compressor accordingly enlarged by the provision of adjustable diiusersl Another way of reducing instability is by the recirculation of compressed medium from the discharge charnber or scroll through the impeller and diiuser 'place in said passages and accordingly the recirculation is effected with little loss of efficiency. VWith the recirculation of compressed medium This instability at tion point to lower iiow values without decreasing the maximum flow and without appreciably affecting .the eiiciencyof the compressor during low ow conditions. The elciency during low flow conditions-may be maintained at a high value by the provision of .variable recirculation nozzles and means for increasing the nozzle areas of the recirculation nozzles with decreasing net flow.

For a better understanding of what I believe to be novel and my invention, attention is dirrected to the following description and the claims appended thereto in connection with the accompanying drawing.

In the drawing Fig. 1 illustrates. a sectional view of a centrifugal compressor such as are used as superchargers on aircraft; Fig. 2 is an enlarged sectional View along lines 2--2 of Fig. 1; Fig. 3 is a sectional view along lines 3 3 of Fig. 2, Fig. 4 is a perspective` view, partly broken away. of the diffuser of Fig. 1;.and Fig. 5 illustrates a sectional view of a modification of my invention.

The centrifugal compressor in Figs. 1 to 4 in elusive comprises a casing having two halves I and 2 secured together by a row of bolts 3 and vhaving inner portions defining an impeller the diffuser' passages remain substantially lled at different load conditions, particularly while the net compressor ow ls reduced.

At low flow a large percentage of the static pressure rise in the compressor occurs beyond the impeller discharge. This large rise makes available aA large pressure ratio for effecting recirculation between the compressor discharge and the point of re-entry into the impeller diffuser passages, that is into the flow passages of the compressor at a point ahead of the diffuser inlet. With such a large pressure ratio a large proportion of the flow may be recirculated. At high load condition, this is, as the maximum diii'user ow' is approached the static pressure rise through the diffuser becomes less and may even become negative at very high flow." Therefore, asfull flow is approached, less and less iiow is recirculated. In other words, the amount of flow recirculated is automatically regulated with changes in net ow through the compressor. An arrangement of this kind moves the pulsachamber and outer portions defining a discharge chamber or scroll 4. The casing half I has a central portion forming an inlet conduitv 5 for conducting medium, such as air, to be com-- pressed to the impeller chamber. The casing half I also forms a anged discharge conduit 6 for discharging compressed medium'to a consumer, not shown. An impeller 'l is' located in the impeller chamber and yhas a plurality oi circumferentially spaced blades t secured to a disk 9 and forming impeller` passages between them. The impeller is secured to an overhang. portion of a shait I8 supported on a bearing iI and may be driven in' known manner by an 'exhaust gas-operated turbine (not shown) with a bucket wheel secured to another overhung portion of the shaft Il). 'I'he compressor casing is supported by a` plurality of bolts I2 on the arms I3 of a spider integral with the casing `of the bearing I/I. Medium impelled by the impeller lspa'ssed through a diffuser Il into the discharge chamber or scroll 4. The dlii'user in the present example has two spaced walls I5 and I6 sup-y ported on shoulders I'I and I8 respectively of the casing halves I and 2. A plurality of circumferentially spaced curved diiuser vanes I9 are located between and integrally formed with the walls` I5, I6. The diffuser is rigidly secured to the casing half 2 by a plurality of circumllo'w load part of the compressed medium from the scroll 4 through a portion of the impeller diffuser passages. In the embodiment of Figs. l to 4 inclusive recirculation is effected at low load through the major portion of the diffuser passages. To this end the walls I5, I6 have openings 22 near the inlets to the diffuser passages, Figs. 2, 4. Recirculating nozzles are associated with these openings. each recirculating nozzle means is formed by three circumferentially spaced curved vanes or blades 23 secured at their ends to angle irons 24, 25,v which latter are attached to the side walls of the diffuser by bolts 2E. The blades 23 have thick inlet portions and sharp edged outlet portions. The outlet edges of the blades are located in the plane of the inner surfaces I5, I6. The outer of the three blades 23 have portions engaging suitably shaped edges'of the respective walls I5, I6. The three blades 23 form two nozzle passages 21 for conducting compressed medium from the scroll to the diffuser passages.

While in the present instance such recirculating nozzles are provided with regard to both side walls I5, I of the diffuser, inother cases only one of said side walls may be provided With recirculating nozzles. With these recirculating nozzles the diffuser passages may be iilled with fluid during low load operation, thereby reducing the Vwith the scroll. Two nozzle passages 29, 30, Fig.

2, are formed through opposite sides of the diffuser vane to establish communication between the opening 28 and the diffuser passages. During operation, compressed medium flows from the discharge chamber 4 through the openings 28 and the nozzle passages 29, 3i! to be discharged 2inv the present example therefrom along the opposite surfaces of the respective diffuser vanes into the diffuser passages. This medium at low net flow fills the passages and also accelerates the-boundary layersl along the diiuser vaines.

The compressor in Fig. 5 comprises a casing having casing halves 3| and 32, corresponding to casing halves I and 2 of Fig. l and enclosing an impeller 33 and a diffuser having `side walls 34 and 35 and a plurality of circumferentially spaced vanes 36, corresponding tothe vanes I9 of Fig. 1. The diffuser is in all respects similar to that of Fig. 1 except that it is not provided with recirculation nozzles. pressed medium from the scroll through a part of the impeller diffuser passages in the present example is accomplished by the provision of openings or nozzles 31 in the casing half 3l, which nozzles are located at a point radially inward from the outlet of the impeller. With sucharrangement the iluid is recirculated through the outer parts of the impeller passages and through the entire diil'user passages. The nozzles 31 when .The recirculation of com-` impeller passages at a; velocity which is substantially the sa'me as the absolute velocity of the air in the particular region of the impeller. The mixing losses produced by mixing of medium flowing through the inletV of the impeller and through the nozzles 31 thereby are kept at a minimum. By moving the point of entry back toward the impeller inlet the pressure drop across the recirculation nozzle becomes greater and accordingly the discharge velocity from the nozzle likewise becomes greater. As the point of reentry is moved from the impeller exit toward the impeller entrance the absolute velocity of the medium becomes smaller. Hence, by placing the re-entry or recirculating nozzles over the open side of the impeller Athe velocity of the air discharged from such nozzles can be readily made the same as the velocity .within the impeller passages near the nozzles.

Therefore, by discharging the medium from the recirculation nozzles in substantially the same direction as the absolute velocity of the medium in the impeller passages at the point of 're-entry, the recirculation may be accomplished without incurring appreciable losses thereby.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A diffuser for centrifugal type compressors including a side wall having crcumferentially spaced openings, a plurality of circumferentially spaced curved vanes secured to the side wall, and a plurality of recirculation nozzles each including circumferentially spaced substantially radial blades disposed in said openings and secured to the side wall.

2. A centrifugal compressor comprising a casing forming an impeller chamber and a discharge scroll, an impeller located in the impeller chamber, a diffuser substantially located in the discharge scroll and having axially spaced side walls and circumferentially spaced vanes secured to the side walls and forming diffuser passages therewith, and means for circulating compressed fluid from the discharge scroll to the inlet portion of lthe diiuser passages, said means comprising recirculating nozzles in the side walls adjacent the inlets of the diiuser passages and transverse passages extending through the radially inner ends of the v-anes and at least one side wall and communicating with the discharge scroll, and nozzles formed in the vanes for receiving compressed fluid from said transverse passages and discharging it into the diifuser passages in substantially the direction of the fluid flow in the diiuser passages.

3. A diffuser arranged to be supported at least partly within the discharge scroll of a centrifugal compressor and having sidewalls lying substantially in axially spaced parallel planes and a plurality of circumferentially spaced vanes secured to said Walls to define therewith radially and tangentially extending diffuser passages, said walls and vanes having a plurality of passages and nozzles arranged to receive fluid from the discharge scroll and to discharge said uid into i each diffuser passage adjacent the inlet thereto properly located and designed will discharge ccmg pressed medium to an intermediate region of the and around substantially the entire periphery of said inlet and in substantially the same dirontion as the fluid flowing through the diffuser passage.

. WENDELL S. THOMPSON. 

